// when animating on canvas, it is best to use requestAnimationFrame instead of setTimeout or setInterval
// not supported in all browsers though and sometimes needs a prefix, so we need a shim
window.requestAnimFrame = (function() {
  return (
    window.requestAnimationFrame ||
    window.webkitRequestAnimationFrame ||
    window.mozRequestAnimationFrame ||
    function(callback) {
      window.setTimeout(callback, 1000 / 60)
    }
  )
})()

// now we will setup our basic variables for the demo
var canvas = document.getElementById('canvas'),
  ctx = canvas.getContext('2d'),
  // full screen dimensions
  cw = window.innerWidth,
  ch = window.innerHeight,
  // firework collection
  fireworks = [],
  // particle collection
  particles = [],
  // starting hue
  hue = 120,
  // when launching fireworks with a click, too many get launched at once without a limiter, one launch per 5 loop ticks
  limiterTotal = 5,
  limiterTick = 0,
  // this will time the auto launches of fireworks, one launch per 80 loop ticks
  timerTotal = 80,
  timerTick = 0,
  mousedown = false,
  // mouse x coordinate,
  mx,
  // mouse y coordinate
  my

// set canvas dimensions
canvas.width = cw
canvas.height = ch

// now we are going to setup our function placeholders for the entire demo

// get a random number within a range
function random(min, max) {
  return Math.random() * (max - min) + min
}

// calculate the distance between two points
function calculateDistance(p1x, p1y, p2x, p2y) {
  var xDistance = p1x - p2x,
    yDistance = p1y - p2y
  return Math.sqrt(Math.pow(xDistance, 2) + Math.pow(yDistance, 2))
}

// create firework
function Firework(sx, sy, tx, ty) {
  // actual coordinates
  this.x = sx
  this.y = sy
  // starting coordinates
  this.sx = sx
  this.sy = sy
  // target coordinates
  this.tx = tx
  this.ty = ty
  // distance from starting point to target
  this.distanceToTarget = calculateDistance(sx, sy, tx, ty)
  this.distanceTraveled = 0
  // track the past coordinates of each firework to create a trail effect, increase the coordinate count to create more prominent trails
  this.coordinates = []
  this.coordinateCount = 3
  // populate initial coordinate collection with the current coordinates
  while (this.coordinateCount--) {
    this.coordinates.push([this.x, this.y])
  }
  this.angle = Math.atan2(ty - sy, tx - sx)
  this.speed = 2
  this.acceleration = 1.05
  this.brightness = random(50, 70)
  // circle target indicator radius
  this.targetRadius = 1
}

// update firework
Firework.prototype.update = function(index) {
  // remove last item in coordinates array
  this.coordinates.pop()
  // add current coordinates to the start of the array
  this.coordinates.unshift([this.x, this.y])

  // cycle the circle target indicator radius
  if (this.targetRadius < 8) {
    this.targetRadius += 0.3
  } else {
    this.targetRadius = 1
  }

  // speed up the firework
  this.speed *= this.acceleration

  // get the current velocities based on angle and speed
  var vx = Math.cos(this.angle) * this.speed,
    vy = Math.sin(this.angle) * this.speed
  // how far will the firework have traveled with velocities applied?
  this.distanceTraveled = calculateDistance(this.sx, this.sy, this.x + vx, this.y + vy)

  // if the distance traveled, including velocities, is greater than the initial distance to the target, then the target has been reached
  if (this.distanceTraveled >= this.distanceToTarget) {
    createParticles(this.tx, this.ty)
    // remove the firework, use the index passed into the update function to determine which to remove
    fireworks.splice(index, 1)
  } else {
    // target not reached, keep traveling
    this.x += vx
    this.y += vy
  }
}

// draw firework
Firework.prototype.draw = function() {
  ctx.beginPath()
  // move to the last tracked coordinate in the set, then draw a line to the current x and y
  ctx.moveTo(this.coordinates[this.coordinates.length - 1][0], this.coordinates[this.coordinates.length - 1][1])
  ctx.lineTo(this.x, this.y)
  ctx.strokeStyle = 'hsl(' + hue + ', 100%, ' + this.brightness + '%)'
  ctx.stroke()

  ctx.beginPath()
  // draw the target for this firework with a pulsing circle
  ctx.arc(this.tx, this.ty, this.targetRadius, 0, Math.PI * 2)
  ctx.stroke()
}

// create particle
function Particle(x, y) {
  this.x = x
  this.y = y
  // track the past coordinates of each particle to create a trail effect, increase the coordinate count to create more prominent trails
  this.coordinates = []
  this.coordinateCount = 5
  while (this.coordinateCount--) {
    this.coordinates.push([this.x, this.y])
  }
  // set a random angle in all possible directions, in radians
  this.angle = random(0, Math.PI * 2)
  this.speed = random(1, 10)
  // friction will slow the particle down
  this.friction = 0.95
  // gravity will be applied and pull the particle down
  this.gravity = 1
  // set the hue to a random number +-20 of the overall hue variable
  this.hue = random(hue - 20, hue + 20)
  this.brightness = random(50, 80)
  this.alpha = 1
  // set how fast the particle fades out
  this.decay = random(0.015, 0.03)
}

// update particle
Particle.prototype.update = function(index) {
  // remove last item in coordinates array
  this.coordinates.pop()
  // add current coordinates to the start of the array
  this.coordinates.unshift([this.x, this.y])
  // slow down the particle
  this.speed *= this.friction
  // apply velocity
  this.x += Math.cos(this.angle) * this.speed
  this.y += Math.sin(this.angle) * this.speed + this.gravity
  // fade out the particle
  this.alpha -= this.decay

  // remove the particle once the alpha is low enough, based on the passed in index
  if (this.alpha <= this.decay) {
    particles.splice(index, 1)
  }
}

// draw particle
Particle.prototype.draw = function() {
  ctx.beginPath()
  // move to the last tracked coordinates in the set, then draw a line to the current x and y
  ctx.moveTo(this.coordinates[this.coordinates.length - 1][0], this.coordinates[this.coordinates.length - 1][1])
  ctx.lineTo(this.x, this.y)
  ctx.strokeStyle = 'hsla(' + this.hue + ', 100%, ' + this.brightness + '%, ' + this.alpha + ')'
  ctx.stroke()
}

// create particle group/explosion
function createParticles(x, y) {
  // increase the particle count for a bigger explosion, beware of the canvas performance hit with the increased particles though
  var particleCount = 30
  while (particleCount--) {
    particles.push(new Particle(x, y))
  }
}

// main demo loop
function loop() {
  // this function will run endlessly with requestAnimationFrame
  requestAnimFrame(loop)

  // increase the hue to get different colored fireworks over time
  hue += 0.5

  // normally, clearRect() would be used to clear the canvas
  // we want to create a trailing effect though
  // setting the composite operation to destination-out will allow us to clear the canvas at a specific opacity, rather than wiping it entirely
  ctx.globalCompositeOperation = 'destination-out'
  // decrease the alpha property to create more prominent trails
  ctx.fillStyle = 'rgba(0, 0, 0, 0.5)'
  ctx.fillRect(0, 0, cw, ch)
  // change the composite operation back to our main mode
  // lighter creates bright highlight points as the fireworks and particles overlap each other
  ctx.globalCompositeOperation = 'lighter'

  var text = '新 年 快 乐'
  ctx.font = '50px sans-serif'
  var textData = ctx.measureText(text)
  ctx.fillStyle = 'rgba(' + parseInt(random(0, 255)) + ',' + parseInt(random(0, 255)) + ',' + parseInt(random(0, 255)) + ',0.3)'
  ctx.fillText(text, cw / 2 - textData.width / 2, ch / 2)

  // loop over each firework, draw it, update it
  var i = fireworks.length
  while (i--) {
    fireworks[i].draw()
    fireworks[i].update(i)
  }

  // loop over each particle, draw it, update it
  var i = particles.length
  while (i--) {
    particles[i].draw()
    particles[i].update(i)
  }

  // launch fireworks automatically to random coordinates, when the mouse isn't down
  if (timerTick >= timerTotal) {
    if (!mousedown) {
      // start the firework at the bottom middle of the screen, then set the random target coordinates, the random y coordinates will be set within the range of the top half of the screen

      for (var h = 0; h < 50; h++) {
        fireworks.push(new Firework(cw / 2, ch / 2, random(0, cw), random(0, ch)))
      }

      timerTick = 0
    }
  } else {
    timerTick++
  }

  // limit the rate at which fireworks get launched when mouse is down
  if (limiterTick >= limiterTotal) {
    if (mousedown) {
      // start the firework at the bottom middle of the screen, then set the current mouse coordinates as the target
      fireworks.push(new Firework(cw / 2, ch / 2, mx, my))
      limiterTick = 0
    }
  } else {
    limiterTick++
  }
}

// mouse event bindings
// update the mouse coordinates on mousemove
canvas.addEventListener('mousemove', function(e) {
  mx = e.pageX - canvas.offsetLeft
  my = e.pageY - canvas.offsetTop
})

// toggle mousedown state and prevent canvas from being selected
canvas.addEventListener('mousedown', function(e) {
  e.preventDefault()
  mousedown = true
})

canvas.addEventListener('mouseup', function(e) {
  e.preventDefault()
  mousedown = false
})

// once the window loads, we are ready for some fireworks!
window.onload = loop
